“Twitter tends to spike really fast, but that spike actually goes down again quite quickly,” ShareThis VP Andy Stevens told me last week. “There’s actually a lot more conversation that takes place across Facebook and Reddit.”

It’s true that the conversation on Facebook and Reddit is slower to hit, but it’s also longer-lasting.

ShareThis says that in its recent study examining social sharing of the new fall TV lineup, Facebook accounted for a massive 78.8 percent of social shares, by far the largest percentage. Reddit had 8.8 percent, while just 8.4 percent of social sharing activity about TV programming occurs on Twitter.

There are a number of reasons for that, Stevens says.

“Not everyone watches TV live anymore …. we use our DVRs, Hulu, or Netflix to watch.”

Besides that, of course, there’s also the fact that Facebook simply has a much bigger audience than Twitter — well over a billion to Twitter’s couple of hundred million (most of whom are overseas). And that the half-life of a tweet is minutes, compared to hours or days on Facebook, and months on sites like Pinterest. So the spike on Facebook, while slower, is much longer, and much bigger.

So Twitter’s immediacy, which is its core strength, could also be its core weakness — from a marketing point of view.

What that means, according to Stevens, is that marketers need to think about more than one channel. And that Facebook, which has almost felt a little ignored by old-school broadcast media in the rush to have real-time second-screen social activity, is a network that might ultimately drive more value.

“Facebook has a much larger userbase, and people are much more active on Facebook,” Stevens told me. “It’s driving a lot more activity.”